Reflective polarizer, fiber, and process for making

ABSTRACT

A fiber comprises a birefringent fibril discontinuous polymeric phase dispersed in a continuous polymeric phase with prescribed matched and mis-matched refractive indices. A diffusely polarizing organic film, optical element, display, and method of making such a film are also disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from provisional application 60/810,888filed Jun. 5, 2006.

FIELD OF THE INVENTION

This invention relates to the field of diffusely reflecting polarizersand polarizing displays and to a fiber useful therein.

BACKGROUND OF THE INVENTION

The performance potential and flexibility of polarized displays,especially those utilizing the electro-optic properties of liquidcrystalline materials, has led to a dramatic growth in the use of thesedisplays for a wide variety of applications. Liquid crystal displays(LCD's) offer the full range from extremely low cost and low powerperformance (e.g. wristwatch displays) to very high performance and highbrightness (e.g. AMLCD's for avionics applications, computer monitorsand HDTV LCD's). Much of this flexibility comes from the light valvenature of these devices, in that the imaging mechanism is decoupled fromthe light generation mechanism. While this is a tremendous advantage, itis often necessary to trade performance in certain categories such asluminance capability or light source power consumption in order tomaximize image quality or affordability. This reduced optical efficiencycan also lead to performance restrictions under high illumination due toheating or fading of the light-absorbing mechanisms commonly used in thedisplays.

In portable display applications such as backlit laptop computermonitors or other instrument displays, battery life is greatlyinfluenced by the power requirements of the display backlight. Thus,functionality must be compromised to minimize size, weight and cost.Avionics displays and other high performance systems demand highluminance but yet place restrictions on power consumption due to thermaland reliability constraints. Projection displays are subject toextremely high illumination levels, and both heating and reliabilitymust be managed. Head mounted displays utilizing polarized light valvesare particularly sensitive to power requirements, as the temperature ofthe display and backlight must be maintained at acceptable levels.

Previously disclosed displays suffer from low efficiency, poor luminanceuniformity, insufficient luminance and excessive power consumption whichgenerates unacceptably high levels of heat in and around the display.Previously disclosed displays also exhibit a non-optimal environmentalrange due to dissipation of energy in temperature sensitive components.Backlight assemblies are often excessively large in order to improve theuniformity and efficiency of the system.

Several areas for efficiency improvement are readily identified.Considerable effort has gone into improving the efficiency of the lightsource (e.g. fluorescent lamps) and optimizing the reflectivity andlight distribution of backlight cavities to provide a spatially uniform,high luminance light source behind the display. Pixel aperture ratiosare made as high as the particular LCD approach and fabrication methodwill economically allow. Where color filters are used, these materialshave been optimized to provide a compromise between efficiency and colorgamut Reflective color filters have been proposed for returning unusedspectral components to a backlight cavity.

When allowed by the display requirements, some improvement can also beobtained by constricting the range of illumination angles for thedisplays via directional techniques.

Even with this previous disclosure optimization, lamp power levels mustbe undesirably high to achieve the desired luminance. When fluorescentlamps are operated at sufficiently high power levels to provide a highdegree of brightness for a cockpit environment, for example, the excessheat generated may damage the display. To avoid such damage, this excessheat must be dissipated. Typically, heat dissipation is accomplished bydirecting an air stream to impinge upon the components in the display.Unfortunately, the cockpit environment contains dirt and otherimpurities which are also carried into the display with the impingingair, if such forced air is even available. Presently available LCDdisplays cannot tolerate the influx of dirt and are soon too dim anddirty to operate effectively.

Another drawback of increasing the power to a fluorescent lamp is thatthe longevity of the lamp decreases dramatically as ever higher levelsof surface luminance are demanded. The result is that aging isaccelerated which may cause abrupt failure in short periods of time whenoperating limitations are exceeded.

Considerable emphasis has also been placed on optimizing the polarizersfor these displays. By improving the pass-axis transmittance(approaching the theoretical limit of 50%), the power requirements havebeen reduced, but the majority of the available light is still absorbed,constraining the efficiency and leading to polarizer reliability issuesin high throughput systems as well as potential image quality concerns.

A number of polarization schemes have been proposed for recapturing aportion of the otherwise lost light and reducing heating in projectiondisplay systems. These include the use of Brewster angle reflections,thin film polarizers, birefringent crystal polarizers and cholestericcircular polarizers. While somewhat effective, these previous disclosureapproaches are very constrained in terms of illumination or viewingangle, with several having significant wavelength dependence as well.Many of these add considerable complexity, size or cost to theprojection system, and are impractical on direct view displays. None ofthese previous disclosure solutions are readily applicable to highperformance direct view systems requiring wide viewing angleperformance.

Also taught in the previous disclosure (U.S. Pat. No. 4,688,897) is thereplacement of the rear pixel electrode in an LCD with a wire gridpolarizer for improving the effective resolution of twisted nematicreflective displays, although this reference falls short of applying thereflective polarizing element for polarization conversion and recapture.The advantages which can be gained by the approach, as embodied in theprevious disclosure, are rather limited. It allows, in principle, themirror in a reflective LCD to be placed between the LC material and thesubstrate, thus allowing the TN mode to be used in reflective mode witha minimum of parallax problems. While this approach has been proposed asa transflective configuration as well, using the wire grid polarizerinstead of the partially-silvered mirror or comparable element, theprevious disclosure does not provide

means for maintaining high contrast over normal lighting configurationsfor transflective displays. This is because the display contrast in thebacklit mode is in the opposite sense of that for ambient lighting. As aresult, there will be a sizable range of ambient lighting conditions inwhich the two sources of light will cancel each other and the displaywill be unreadable. A further disadvantage of the previous disclosure isthat achieving a diff-usely reflective polarizer in this manner is notat all straightforward, and hence the reflective mode is most applicableto specular, projection type systems.

Disclosed in the previous disclosure (U.S. Pat. No. 2,604,817) and laterin the previous disclosure(U.S. Pat. No. 5,999,239) is a means toproduce a diffusely reflective polarizer utilizing polymeric fibersdispersed in a continuous polymer matrix. Typical monofilamentbirefringent fibers(ex, polyester) were demonstrated to create such adiffuse reflective polarizer in (U.S. Pat. No. 2,604,817). These fibersare embedded into an isotropic polymer matrix. The manufacturability andoptical performance of such a reflective polarizer utilizing even thesmallest typical monolithic birefringent fibers, however, is notsufficient enough to enable such a diffuse reflective polarizer to becost effective. It is, therefore, an object of the present invention toimprove the optical efficiency of polarized displays, especially directview liquid crystal displays (LCDs) and to simplify manufacture andreduce the costs thereof.

It is a further object of the present invention to provide thisefficiency increase while retaining wide viewing angle capability andminimize the introduction of chromatic shifts or spatial artifacts.

It is a further object of the present invention to reduce the absorptionof light by polarized displays, minimizing heating of the displays anddegradation of the display polarizers.

It is a further object of the present invention to provide an LCD havingincreased display brightness.

It is yet a further object of the present invention to reduce the powerrequirements for LCD backlight systems.

It is yet a further object of the present invention to improve displaybacklight uniformity without sacrificing performance in other areas.

It is still a further object of the present invention to achieve theseobjects by using a process that enables a cost-effective means toproduce an efficient diffusely reflective polarizer for use in LCDbacklight systems.

Cost-effectiveness is achieved by utilizing a unique island-in-the seafiber design and a unique extrusion process to create a diffuselyreflective polarizer.

SUMMARY OF THE INVENTION

The invention provides a fiber comprising a birefringent fibrildiscontinuous polymeric phase dispersed in a continuous polymeric phasewherein the refractive indices of the discontinuous and continuousphases in the X and Y directions are substantially matched to eachother, the refractive indices of the discontinuous and continuous phasesin the X and Y directions are substantially mismatched from therefractive index of the fibril discontinuous polymeric phase in the Zdirection, and wherein the extrusion melting temperature of thecontinuous phase is less than the onset melting range of thediscontinuous phase.

The invention also includes a process for making the fiber and anoptical element employing the fiber. The invention enables one toimprove the optical efficiency of polarized displays, especially directview liquid crystal displays (LCDs) and to simplify manufacture andreduce the costs thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an end view of an island-in-the-sea fiber (10) with acontinuous phase (20) and internal fibrils (discontinuous phase) 30.

FIG. 1B is a 3D view of an island-in-the-sea fiber 10 with theprojection of the fibril 30 is the length direction with a continuousphase polymer (sea) 20 between the fibrils. FIG. 2 is a perspective viewof the island-in-the-sea fiber 10 with fibril 30 and sea polymer 20. Thesea polymer and fibrils comprise 3 dimensions of Refractive index. Thefiber is stretched in the length (Z) direction and therefore there is anordinary refractive index for the sea polymer and fibrils in the X and Yplane and an extraordinary index in the length (Z) direction as shown bysymbol 40 & 41. The ordinary and extraordinary indices of the fibril maybe different than the sea polymer indices.

FIG. 3A is a circular island-in-the-sea fiber 10 with elliptical fibrils31.

FIG. 3B is a circular island-in-the-sea fiber 10 with circular fibrils30

FIG. 3C is an elliptical island-in-the-sea fiber 11 with radial fibrils30

FIG. 3D is an elliptical island-in-the-sea fiber 11 with mixed shapesand size fibrils 30 and 31

FIG. 3E is a rectilinear shaped island-in-the-sea fiber 12 withelliptical shaped fibrils 31

FIG. 3F is a rectilinear shaped island-in-the-sea fiber 12 with randomrectilinear shaped fibrils 32

FIG. 3G is a circular shaped island-in-the-sea fiber 10 with triangularshaped fibrils 33.

FIG. 4A is a section view of several island-in-the-sea fiber 10 enteringthe feed port of an extruder barrel 50 prior to melting the sea polymer.

FIG. 4B is a section view of the melt stream in a pipe exiting theextruder 60 of several fibrils 31 that have been dispersed into themelted sea polymer 20.

FIG. 5A is a cross machine cross section of the composite sheet (1)showing the fibrils (10) dispersed throughout.

FIG. 5B is a down machine cross section of the composite sheet (1)showing fibrils 31 dispersed and aligned throughout.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for producing a diffuselyreflecting polarizer film made up of a composite of birefringentpolymeric fibrils dispersed in an isotropic polymeric phase. Thebirefringent fibrils are created by producing multi-componentisland-in-the-sea fibers whereby the birefringent fibrils are islands ina sea of a continuous polymeric phase and wherein the refractive indicesof the continuous phase in the X and Y directions (see FIG. 2) aresubstantially matched and wherein the extrusion melting temperature ofthe continuous phase is less than the onset melting range of thediscontinuous phase. These fibers are then cut to short lengths andeither solely extruded or extruded with additional resin pelletscomprising either the same polymer as that of the continuous polymericphase of the island-in-the sea fiber or a polymer with very similaroptical and processing properties. The extrusion is done at atemperature sufficient to melt the continuous polymeric phase andadditional resin pellets but not high enough to initiate melting of thebirefringent fibrils. The fibrils are mixed and uniformly dispersed inthe melted continuous polymeric phase. The melted mix is then pumpedthrough a filming extrusion die with narrow enough die lands to producehigh pressure and high shear forces on the fibrils thus orienting andaligning the fibrils in the machine direction. The extruded film is thencooled and a resulting diffusely reflecting polarizer film is formed.

Definitions:

The term fibril is defined as a material phase in a fiber that isdiscontinuous in the cross sectional plane of the fiber but eithercontinuous in the fiber length direction or otherwise elongated to adimension in the fiber length direction at least 100 times greater thanthe largest dimension in the cross section plane.

Extrusion melting temperature is defined here as a temperature at whichthe viscosity of the melted polymer is in a range that enablesprocessing at reasonable pressures, and will be defined here as 100degrees C. above the glass transition temperature of the polymer.

Onset melting temperature is defined here as the temperature near themelting point of the polymer at which thermal energy is first observedto be seen imparted to the birefringent polymer fibril during a standarddifferential scanning calorimeter measurement.

In order to make the fiber composite film of the present inventioneffective as a reflective polarizer it is desirable to create many smallfibrils within a fiber such that many more optical interfaces can becreated in a given thickness of film when dispersed by the process ofthe present invention into a composite film. Processes to create fiberswith many small fibrils, also known as, island-in-the sea fiber makingprocesses are well known in the trade. In particular the processes asdescribed in U.S. Pat. Nos. 5,162,074 and 5,466,410 utilizingphoto-etched plates to control flow of the different polymer melts inthe multi-component fiber are very suitable. The cross sectional shapeof the fibers can be of any geometry such as circular, rectilinear,elliptical, triangular, tri-lobal, or trapezoidal. Typically the fibercross sectional shape will be circular or elliptical with the mostcommon cross sectional shape being circular. Similarly, the crosssectional shape of the fibrils can be of any geometry such as circular,rectilinear, elliptical, triangular, tri-lobal, or trapezoidal. Again,typically the fibril cross sectional shape will be circular orelliptical with the most common cross sectional shape being circular.

It should be noted that polymeric surfactants also referred to ascompatibilizers may be added to either one or both polymer of thediscontinuous and continuous phases of the fibers. Typical materials mayinclude blocked or grafted copolymers where segments of the copolymermatches that of either or both the discontinuous and or continuousphases in the polymeric fiber. The copolymers may be added in a weightratio of 0.05 to 10 percent. This range may vary depending on the degreeof substitution on the copolymer.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the designs and methodsdisclosed herein may be made without departing from the scope of theinvention, which is defined in the appended claims.

Fiber description

Many items are made from synthetic fibers. Conventionally, two processesare used to manufacture synthetic fibers: a solution spinning processand a melt spinning process. The solution spinning process is generallyused to form acrylic fibers, while the melt spinning process isgenerally used to form nylon fibers, polyester fibers, polypropylenefibers, and other similar type fibers. As is well known a polyesterfiber comprises a long-chain synthetic polymer having at least 85percent by weight of an ester of a substituted aromatic carboxylic acidunit.

The melt spinning process is of particular interest as since a largeportion of the synthetic fibers that are used in the textile industryare manufactured by this technique and the process is ubiquitous atproduction scale. Also, since the present invention also requires uniquedown stream extrusion processing of the fibers to produce a compositefilm with oriented fibrils, melt spun fibers are desirable. The meltspinning process generally involves passing a molten polymeric materialthrough a device that is known as a spinneret to thereby form aplurality of individual synthetic fibers. Once formed, the syntheticfibers are typically collected into a strand or cut into staple fibers.Synthetic fibers are typically used to make knitted, woven, or non-wovenfabrics, or alternatively, synthetic fibers can be spun into a yarn tobe used thereafter in a weaving or a knitting process to form asynthetic fabric. Multi-component fibrils have been well demonstrated inprevious disclosures. Such fibers comprise two or more polymers andtypically are designed to either split apart due to incompatibility ofthe polymers or one polymer is dissolved in solvent such that smallerfibrils of the other polymer are left. This method results in muchsmaller fibers or fibrils than can be traditionally produces viamono-component fiber processes and offers a wider range of finalproperties of the fiber-based article in which the fibers are used. Thepresent invention relates to a multi-component fiber having both abirefringent polymeric fibril component as well as a continuouspolymeric phase component with a melt processing temperature lower thanthe onset melting temperature of the birefringent fibril.

The birefringent fibrils in the island-in-the-sea fiber of the presentinvention can comprise any polymer in the general class of polyesters.Typical polyesters for such use can be polyethylene(terephlatate),polyethylene(naphthalate), or any copolymers of either. The mostsuitable polyester for the birefringent fibril ispolyethylene(naphthalate).

The continuous polymeric phase in the island-in-the-sea fiber of thepresent invention can comprise any polymer in the general classes ofpolyesters, acrylics, or olefins. Typical polymers for such use can bepolyethylene(terephlatate), poly(methyl-methacrylate),poly(cyclo-olefin), or any copolymers of either. The most suitablepolymers for the continuous phase is poly(1,4-cyclohexylene dimethyleneterephthalate) or poly(ethylene-terephthalate/isophthalate) copolymer.

As mentioned previously the extrusion melting temperature of thecontinuous polymeric phase of the fibers should be less than the onsetmelting temperature of the birefringent fibrils. Typically thisdifference will be greater than 10° C. but is preferred to be greaterthan 40° C. Most preferably the extrusion melting temperature of thecontinuous polymeric phase is greater than 75° C. below the onsetmelting temperature of the birefringent fibrils.

The island-in-the sea fibers of the present invention are cold drawnafter being melt spun as is typical for such a fiber process. The colddraw is done with the fibers heated to just above the glass transitiontemperature (Tg) of the fibrils polymer. Typically the cold draw is doneat 2 to 20° C. above Tg.

The amount of draw or draw ratio, which is the ratio by which the fiberis lengthened relative to its initial length, is important in attaininga high level of birefringence of the fibril. This is important as itcreates a large difference in the Z direction (see FIG. 2) extraordinaryindex of the fibril and the eventual Z direction (see FIG. 5B) ordinaryindex of the continuous phase of the composite film. The continuousphase of the fiber is melt relaxed during film processing and thereforeretains the ordinary index in the Z direction of the final compositefilm resulting in an isotropic continuous phase. The large difference inZ direction index of the fibril and the continuous phase is desired asit results in a high degree of reflection of light that passes throughthe film that is approaching the film orthogonal to the film surface andis linearly polarized parallel to the length of the fibril. The drawratio should be greater than 2 to 1 and preferably greater than 3 to 1.Most preferably the draw ratio is greater than 3.5 to 1 to maximize thedegree of crystallinity and thus birefringence of the fibrils.

The continuous polymeric phase may also become birefringent in thedrawing process but this is not critical. Any birefringence of thecontinuous phase polymer will be eliminated during the subsequentextrusion process when making the composite polarizing film. Thereforedrawing temperature is only critical for the continuous phase polymer tothe degree that the polymer will stretch at the draw temperature withoutcracking and/or sticking to the draw rollers.

As mentioned previously, a large number of smaller fibrils in the fibersis preferable as this will ultimately result in many more opticalinterfaces in the final composite film reflective polarizer. The numberof fibrils in the fiber is determined by the design of the spin pack.For a given spin pack design the size of the fibrils is then determinedby the relative weight ratio of fibril polymer to continuous phasepolymer when melt spinning. Typical weight ratios of fibril polymer tocontinuous phase polymer is less than 2 to 1 and preferably less than0.8 to 1. Most preferably the weight ratio of fibril polymer tocontinuous phase polymer is less than 0.3 to one.

As previously mentioned the size, as measured by cross sectional area,of the fibrils is important because more smaller fibrils can be packedinto a fiber lending to more optical interfaces. Typical cross sectionalareas of fibrils is less than 3.0 square microns. Preferably the crosssectional area is less than 0.6 square microns and most suitably thecross sectional area of the fibrils is less than 0.2 square microns.

The number of fibrils that are in the fiber is important as discussedpreviously and is determined by the spin pack design. Typically thenumber of fibrils in a fiber is greater than 50. Preferably the numberof fibrils is greater than 500. Fibers with greater than 1000 fibrilshave been demonstrated and are most preferred.

The X and Y directions are orthogonal and in the plane of the crosssection of the fibers and fibrils, see FIG. 2. The refractive indices inthe X and Y directions of the fibrils (ordinary indices) and of thecontinuous polymeric phase of the fibers should be substantially matched(i.e., differ by less than about 0.05) in either the X or Y axes (seeFIG. 2). The refractive indices in the X and Y directions of the fibrils(ordinary indices) and of the continuous polymeric phase of the fibersare substantially mismatched (i.e., differ by more than about 0.07) fromthe refractive index of fibrils in the Z axis (see FIG. 2). Preferably,the indices of refraction of the continuous and discontinuous phasesdiffer by less than about 0.03 in the match direction, and mostpreferably, less than about 0.02. Preferably, the indices of refractionof the fibrils(ordinary indices) and of the continuous polymeric phaseof the fibers differ from that of the index of refraction of the fibrilsin the Z direction by more than 0.1, and most preferably by more than0.2.

Filming Process Description

The first step to converting the fibers described previously into adiffusely reflective polarizing film is to cut the fibers into shortlengths. This is important as fibers with shorter aspect ratios, definedas length of fibers divided by cross sectional area, can be dispersed inthe melted continuous polymeric phase and be oriented through a shearforce field much more readily. Typically the fibers are cut to a lengthless than 5 mm. Preferably the fibers are cut to a length less than 1 mmand most suitably the fibers are cut to a length of less than 0.4 mm.

Next the cut fibers are fed into the feed port of a typical single screwor twin screw extruder and processed at the extrusion meltingtemperature of the continuous polymeric phase of the fibers. The crosssection of the fibers entering the feed port are illustrated in FIG. 4A.The fibrils will not melt at this temperature as the continuous polymeris chosen as to have an extrusion melting temperature below the onsetmelting temperature of the fibrils. Typically the extrusion meltingtemperature will be more than 10 C. lower than the onset meltingtemperature of the fibrils. Preferably the extrusion temperature will bemore than 40 C. lower than the onset melting temperature of the fibrils.Most suitably the extrusion temperature will be more than 75 C lowerthan the onset melting temperature of the fibrils.

The fibrils are already wetted out buy the continuous polymeric phase bythe nature of the fiber design. This results in very good dispersionquality of the fibrils in the molten continuous phase polymer as aresult of further mixing in the extruder or subsequently to the extrudervia any know melt mixing devices.

FIG. 4B illustrates the fibrils being dispersed in the molten mix via across section of a pipe exiting the extruder or downstream mixingdevice.

Next the dispersed mixture is subjected to high shear forces via pumpingof the mixture through small die gaps in extrusion filming die. Thesehigh shear forces result in the fibrils being oriented parallel to eachother with the length direction of the fibrils parallel to the flowdirection through the die. This results in the composite film having thefibrils aligned parallel to each other and parallel to the machinedirection of the film. The high shear forces are created by attainingmelt pressures in the die greater than 1000 psi. Preferably the diepressures are greater than 2000 psi and most suitably the die pressuresare greater than 3000 psi. These high shear forces result in the fibrilsbeing aligned with the machine direction of the film such that the anglebetween the Z direction axis, see FIG. 2, and the machine direction ofthe film is less than 45 degrees. Preferably the angle between the Zdirection axis and the machine direction of the film is less than 15degrees and most suitably the angle between the Z direction axis and themachine direction of the film is less than 5 degrees.

The aligned fibrils are illustrated in FIGS. 5A and 5B showing the crossmachine direction and down machine direction section views of thecomposite film.

The continuous phase of the dispersed mixture in the final compositefilm comprises at least the continuous polymeric phase of the fibers.Additionally the dispersed mixture can comprise any additional resin orpolymer that is added to the fibers in the extrusion process. This addedpolymer must meet all of the requirements of the continuous phasepolymer of the fiber and can comprise the same resins that have beendescribed for the fiber continuous phase.

The indices of refraction of the continuous and discontinuous phases ofthe composite film are substantially matched (i.e., differ by less thanabout 0.05) along at least a first of three mutually orthogonal axes (Xaxis in FIG. 5A), and are substantially mismatched (i.e., differ by morethan about 0.07) along a second of three mutually orthogonal axes (Zaxis in FIG. 5B). Preferably, the indices of refraction of thecontinuous and discontinuous phases differ by less than about 0.03 inthe match direction, and most preferably, less than about 0.02. Theindices of refraction of the continuous and discontinuous phasespreferably differ in the mismatch direction by at least about 0.07, morepreferably, by at least about 0.1, and most preferably, by at leastabout 0.2.

The mismatch in refractive indices along a particular axis has theeffect that incident light polarized along that axis will besubstantially scattered, resulting in a significant amount ofreflection. By contrast, incident light polarized along an axis in whichthe refractive indices are matched will be spectrally transmitted orreflected with a much lesser degree of scattering. This effect can beutilized to make a variety of optical devices, including reflectivepolarizers and mirrors.

Effect of Index Match/Mismatch

The magnitude of the index match or mismatch along a particular axisdirectly affects the degree of scattering of light polarized along thataxis. In general, scattering power varies as the square of the indexmismatch. Thus, the larger the index mismatch along a particular axis,the stronger the scattering of light polarized along that axis.Conversely, when the mismatch along a particular axis is small, lightpolarized along that axis is scattered to a lesser extent and is therebytransmitted specularly through the volume of the body.

Skin Layers

A layer of material which is substantially free of a discontinuous phasemay be disposed on one or both major surfaces of the composite film,i.e., the extruded composite the discontinuous phase and the continuousphase. The composition of the layer, also called a skin layer, may bechosen, for example, to protect the integrity of the discontinuous phasewithin the extruded blend, to add mechanical or physical properties tothe final film or to add optical functionality to the final film.Suitable materials of choice may include the material of the continuousphase or the material of the discontinuous phase.

A skin layer or layers may also add physical strength to the resultingcomposite or reduce problems during processing, such as, for example,reducing the tendency for the film to split during the orientationprocess. Skin layer materials which remain amorphous may tend to makefilms with a higher toughness, while skin layer materials which aresemi-crystalline may tend to make films with a higher tensile modulus.Other functional components such as antistatic additives, UV absorbers,dyes, antioxidants, and pigments, may be added to the skin layer,provided they do not substantially interfere with the desired opticalproperties of the resulting product.

The skin layers may be applied to one or two sides of the extruded blendat some point during the extrusion process, i.e., before the extrudedblend and skin layer(s) exit the extrusion die. This may be accomplishedusing conventional coextrusion technology, which may include using athree-layer coextrusion die. Lamination of skin layer(s) to a previouslyformed film of an extruded blend is also possible. Total skin layerthicknesses may range from about 2% to about 50% of the total filmthickness.

A wide range of polymers are suitable for skin layers. Predominantlyamorphous polymers include copolyesters based on one or more ofterephthalic acid, 2,6-naphthalene dicarboxylic acid, isophthalic acidphthalic acid, or their alkyl ester counterparts, and alkylene diols,such as ethylene glycol. Examples of semicrystalline polymers are2,6-polyethylene naphthalate, polyethylene terephthalate, and nylonmaterials.

Antireflection Layers

The films and other optical devices made in accordance with theinvention may also include one or more anti-reflective layers. Suchlayers, which may or may not be polarization sensitive, serve toincrease transmission and to reduce reflective glare. An anti-reflectivelayer may be imparted to the resulting film of the of the presentinvention through appropriate surface treatment, such as coating orsputter etching.

In some embodiments of the present invention, it is desired to maximizethe transmission and/or minimize the specular reflection for certainpolarizations of light. In these embodiments, the optical body maycomprise two or more layers in which at least one layer comprises ananti-reflection system in close contact with a layer providing thecontinuous and discontinuous phases. Such an anti-reflection system actsto reduce the specular reflection of the incident light and to increasethe amount of incident light that enters the portion of the bodycomprising the continuous and discontinuous layers. Such a function canbe accomplished by a variety of means well known in the art. Examplesare quarter wave anti-reflection layers, two or more layeranti-reflective stack, graded index layers, and graded density layers.Such antireflection functions can also be used on the transmitted lightside of the body to increase transmitted light if desired.

More Than Two Phases

The composite films made in accordance with the present invention mayalso consist of more than two phases. Thus, for example, an opticalmaterial made in accordance with the present invention can consist oftwo different discontinuous phases within the continuous phase. Thesecond discontinuous phase could be randomly or non-randomly dispersedthroughout the fibrils or can be a separate discontinuous phase from thefibrils, and can be aligned along a common axis.

Composite films made in accordance with the present invention may alsoconsist of more than one continuous phase. Thus, in some embodiments,the optical body may include, in addition to a first continuous phaseand a discontinuous phase, a second phase which is co-continuous in atleast one dimension with the first continuous phase

Multilayer Combinations

If desired, one or more sheets of a continuous/disperse phase film madein accordance with the present invention may be used in combinationwith, or as a component in, a multilayered film (i.e., to increasereflectivity). Suitable multilayered films include those of the typedescribed in WO 95/17303 (Ouderkirk et al.). In such a construction, theindividual sheets may be laminated or otherwise adhered together or maybe spaced apart with the polymeric sheet of this invention. If theoptical thicknesses of the phases within the sheets are substantiallyequal (that is, if the two sheets present a substantially equal andlarge number of scatterers to incident light along a given axis), thecomposite will reflect, at somewhat greater efficiency, substantiallythe same band width and spectral range of reflectivity (i.e., “band”) asthe individual sheets. If the optical thicknesses of phases within thesheets are not substantially equal, the composite will reflect across abroader band width than the individual phases. A composite combiningmirror sheets with polarizer sheets is useful for increasing totalreflectance while still polarizing transmitted light.

Additives

The composite films of the present invention may also comprise othermaterials or additives as are known to the art. Such materials includepigments, dyes, binders, coatings, fillers, compatibilizers,antioxidants (including sterically hindered phenols), surfactants,antimicrobial agents, antistatic agents, flame retardants, foamingagents, lubricants, reinforcers, light stabilizers (including UVstabilizers or blockers), heat stabilizers, impact modifiers,plasticizers, viscosity modifiers, and other such materials.Furthermore, the films and other optical devices made in accordance withthe present invention may include one or more outer layers which serveto protect the device from abrasion, impact, or other damage, or whichenhance the processability or durability of the device.

Suitable lubricants for use in the present invention include calciumstearate, zinc stearate, copper stearate, cobalt stearate, molybdenumneodocanoate, and ruthenium (III) acetylacetonate.

Antioxidants useful in the present invention include4,4′-thiobis-(6-t-butyl-m-cresol),2,2′-methylenebis-(4-methyl-6-t-butyl-butylphenol),octadecyl-3,5-di-t-butyl-4-hydroxyhydrocinnamate, bis-(2,4-di-t-butylphenyl)pentaerythritol diphosphite, Irganox™ 1093(1979)(((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl)-dioctadecylester phosphonic acid), Irganox™ 1098(N,N′-1,6-hexanediylbis(3,5-bis(1,1-dimethyl)-4-hydroxy-benzenepropanamide),Naugaard™ 445 (aryl amine), Irganox™ L 57 (alkylated diphenylamine),Irganox™ L 115 (sulfur containing bisphenol), Irganox™ LO 6 (alkylatedphenyl-delta-napthylamine), Ethanox 398 (flourophosphonite), and2,2′-ethylidenebis(4,6-di-t-butylphenyl)fluorophosnite.

A group of antioxidants that are especially preferred are stericallyhindered phenols, including butylated hydroxytoluene (BHT), Vitamin E(di-alphatocopherol), Irganox™ 1425WL(calciumbis-(O-ethyl(3,5-di-t-butyl-4hydroxybenzyl))phosphonate), Irganox™ 1010(tetrakis(methylene(3,5,di-t-butyl-4-hydroxyhydrocinnamate))methane),Irganox™ 1076 (octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate),Ethanox™ 702 (hindered bis phenolic), Etanox 330 (high molecular weighthindered phenolic), and Ethanox™ 703 (hindered phenolic amine).

Dichroic dyes are a particularly useful additive in some applications towhich the optical materials of the present invention may be directed,due to their ability to absorb light of a particular polarization whenthey are molecularly aligned within the material. When used in a film orother material which predominantly scatters only one polarization oflight, the dichroic dye causes the material to absorb one polarizationof light more than another. Suitable dichroic dyes for use in thepresent invention include Congo Red (sodiumdiphenyl-bis-oc-naphthylamine sulfonate), methylene blue, stilbene dye(Color Index (CI)=620), and 1,1′-diethyl-2,2′-cyanine chloride (CI=374(orange) or CI=518 (blue)). The properties of these dyes, and methods ofmaking them, are described in E. H. Land, Colloid Chemistry (1946).These dyes have noticeable dichroism in polyvinyl alcohol and a lesserdichroism in cellulose. A slight dichroism is observed with Congo Red inPEN.

Other suitable dyes include the following materials: [CHEM-1] Theproperties of these dyes, and methods of making them, are discussed inthe Kirk Othmer Encyclopedia of Chemical Technology, Vol. 8, pp. 652-661(4th Ed. 1993), and in the references cited therein.

When a dichroic dye is used in the optical bodies of the presentinvention, it may be incorporated into either the continuous ordiscontinuous phase. However, it is preferred that the dichroic dye isincorporated into the discontinuous phase.

Dychroic dyes in combination with certain polymer systems exhibit theability to polarize light to varying degrees. Polyvinyl alcohol andcertain dichroic dyes may be used to make films with the ability topolarize light. Other polymers, such as polyethylene terephthalate orpolyamides, such as nylon-6, do not exhibit as strong an s ability topolarize light when combined with a dichroic dye. The polyvinyl alcoholand dichroic dye combination is said to have a higher dichroism ratiothan, for example, the same dye in other film forming polymer systems. Ahigher dichroism ratio indicates a higher ability to polarize light.

Molecular alignment of a dichroic dye within a composite film made inaccordance with the present invention is preferably accomplished bystretching the composite film after the dye has been incorporated intoit. However, other methods may also be used to achieve molecularalignment. Thus, in one method, the dichroic dye is crystallized, asthrough sublimation or by crystallization from solution, into a seriesof elongated notches that are cut, etched, or otherwise formed in thesurface of a film., either before or after the composite film has beenoriented. The treated surface may then be coated with one or moresurface layers, may be incorporated into a polymer matrix or used in amultilayer structure, or may be utilized as a component of anotheroptical body. The notches may be created in accordance with apredetermined pattern or diagram, and with a predetermined amount ofspacing between the notches, so as to achieve desirable opticalproperties.

In a related embodiment, the dichroic dye may be disposed within one ormore hollow fibers or other conduits, either before or after the hollowfibers or conduits are disposed within the composite film. The hollowfibers or conduits may be constructed out of a material that is the sameor different from the surrounding material of the composite film.

In yet another embodiment, the dichroic dye is disposed along the layerinterface of a multilayer construction, as by sublimation onto thesurface of a layer before it is incorporated into the multilayerconstruction. In still other embodiments, the dichroic dye is used to atleast partially backfill the voids in a microvoided film made inaccordance with the present invention.

Functional Layers

Various functional layers or coatings may be added to the compositefilms of the present invention to alter or improve their physical orchemical properties, particularly along the surface of the film. Suchlayers or coatings may include, for example, slip agents, low adhesionbackside materials, conductive layers, antistatic coatings or films,barrier layers, flame retardants, UV stabilizers, abrasion resistantmaterials, optical coatings, or substrates designed to improve themechanical integrity or strength of the film or device.

The films of the present invention may be given good slip properties bytreating them with low friction coatings or slip agents, such as polymerbeads coated onto the surface. Alternately, the morphology of thesurfaces of these materials may be modified, as through manipulation ofextrusion conditions, to impart a slippery surface to the film; methodsby which surface morphology may be so modified are described in U.S.Ser. No. 08/612,710.

In some applications, as where the composite film of the presentinvention are to be used as a component in adhesive tapes, it may bedesirable to treat the films with low adhesion backsize (LAB) coatingsor films such as those based on urethane, silicone or fluorocarbonchemistry. Films treated in this manner will exhibit proper releaseproperties towards pressure sensitive adhesives (PSAs), thereby enablingthem to be treated with adhesive and wound into rolls. Adhesive tapesmade in this manner can be used for decorative purposes or in anyapplication where a diffusely reflective or transmissive surface on thetape is desirable.

The films and optical devices of the present invention may also beprovided with one or more conductive layers. Such conductive layers maycomprise metals such as silver, gold, copper, aluminum, chromium,nickel, tin, and titanium, metal alloys such as silver alloys, stainlesssteel, and intone, and semiconductor metal oxides such as doped andundoped tin oxides, zinc oxide, and indium tin oxide (ITO).

The composite film of the present invention may also be provided withantistatic coatings or films. Such coatings or films include, forexample, V 2 O 5 and salts of sulfonic acid polymers, carbon or otherconductive metal layers.

The optical films and devices of the present invention may also beprovided with one or more barrier films or coatings that alter thetransmissive properties of the optical film towards certain liquids orgases. Thus, for example, the devices and films of the present inventionmay be provided with films or coatings that inhibit the transmission ofwater vapor, organic solvents, O 2, or CO 2 through the film. Barriercoatings will be particularly desirable in high humidity environments,where components of the film or device would be subject to distortiondue to moisture permeation.

The composite films of the present invention may also be treated withflame retardants, particularly when used in environments, such as onairplanes, that are subject to strict fire codes. Suitable flameretardants include aluminum trihydrate, antimony trioxide, antimonypentoxide, and flame retarding organophosphate compounds.

The composite film of the present invention may also be provided withabrasion-resistant or hard coatings, which will frequently be applied asa skin layer. These include acrylic hardcoats such as Acryloid A-11 andParaloid K-120N, available from Rohm & Haas, Philadelphia, Pa.; urethaneacrylates, such as those described in U.S. Pat. No. 4,249,011 and thoseavailable from Sartomer Corp., Westchester, Pa.; and urethane hardcoatsobtained from the reaction of an aliphatic polyisocyanate (e.g.,Desmodur N-3300, available from Miles, Inc., Pittsburgh, Pa.) with apolyester (e.g., Tone Polyol 0305, available from Union Carbide,Houston, Tex.).

The composite film of the present invention may further be laminated torigid or semi-rigid substrates, such as, for example, glass, metal,acrylic, polyester, and other polymer backings to provide structuralrigidity, weatherability, or easier handling. For example, the compositefilm of the present invention may be laminated to a thin acrylic ormetal backing so that it can be stamped or otherwise formed andmaintained in a desired shape. For some applications, such as when theoptical film is applied to other breakable backings, an additional layercomprising PET film or puncture-tear resistant film may be used.

The composite film and devices of the present invention may also beprovided with shatter resistant films and coatings. Films and coatingssuitable for this purpose are described, for example, in publications EP592284 and EP 591055, and are available commercially from 3M Company, StPaul, Minn.

Various optical layers, materials, and devices may also be applied to,or used in conjunction with, the films of the present invention forspecific applications. These include, but are not limited to, magneticor magneto-optic coatings or films; liquid crystal panels, such as thoseused in display panels and privacy windows; photographic emulsions;fabrics; prismatic films, such as linear Fresnel lenses; brightnessenhancement films; holographic films or images; embossable films;anti-tamper films or coatings; IR transparent film for low emissivityapplications; release films or release coated paper; and polarizers ormirrors.

Multiple additional layers on one or both major surfaces of thecomposite film are contemplated, and can be any combination ofaforementioned coatings or films. For example, when an adhesive isapplied to the composite film, the adhesive may contain a white pigmentsuch as titanium dioxide to increase the overall reflectivity, or it maybe optically transparent to allow the reflectivity of the substrate toadd to the reflectivity of the composite film.

In order to improve roll formation and convertibility of the film, thecomposite film of the present invention may also comprise a slip agentthat is incorporated into the film or added as a separate coating. Inmost applications, slip agents will be added to only one side of thefilm, ideally the side facing the rigid substrate in order to minimizehaze.

Thickness of Composite Film

The thickness of the composite film is also an important parameter whichcan be manipulated to affect reflection and transmission properties inthe present invention. As the thickness of the composite film increases,diffuse reflection also increases, and transmission, both specular anddiffuse, decreases. Thus, while the thickness of the composite film willtypically be chosen to achieve a desired degree of mechanical strengthin the finished product, it can also be used to directly to controlreflection and transmission properties. Thickness can also be utilizedto make final adjustments in reflection and transmission properties ofthe composite film. Thus, for example, in film applications, the deviceused to extrude the film can be controlled by a downstream opticaldevice which measures transmission and reflection values in the extrudedfilm, and which varies the thickness of the film (i.e., by adjustingextrusion rates or changing casting wheel speeds) so as to maintain thereflection and transmission values within a predetermined range.

EXAMPLE 1

Polyethylene(naphthalate), PEN (VFR-40102 from M&G Group) was firstdried in a desicant dryer at 140 C for 12 hours. Also, an isophthalicacid modified Co-PET, Crystar® Merge 3991 by DuPont was dried in adesicant dryer at 55 C for 12 hours.

These polymers were then fed into two separate extruders and meltextruded at 300° C. and 270° C., respectively.

Island in the sea fibers were produced by feeding the two melt streamsinto a specially designed spinneret that created 1410 fibrils withineach fiber. The Co-PET was fed as the continuous phase of the fiber andthe PEN was fed as the discontinuous fibrils. 72 fibers were producedsimultaneously by the spinner. The fibers were air cooled upon exitingthe orifices of the spinneret and then heated and stretched 4 timestheir original length at a temperature of 120° C. The final diameter ofthe fibers was nominally 40 μm.

The fibers were wound on bobbins. The fibers were then unwound from thebobbins and fed into a cutter and cut to nominally 0.25 mm in length.The cut fibers were then dried in a desiccant dryer at 55° C. for 12hours. Also, neat Co-PET 3991 pellets were dried in a desiccant dryer at55° C. for 12 hours. The dried cut fibers were then dry blended with thedried Co-PET pellets at a 15 to 85 ratio, respectively. The blend wasfed into a 19mm diameter twin screw extruder where it was melted, mixedand extruded. The melted extrudate was then pumped through an extrusiondie with narrow die slots at an inlet pressure of 2500 psi. The fibrilswithin the melted resin substantially aligned in the machine directiondue to high shear stresses in the flow. A 7″ wide film was extruded fromthe extrusion die and cooled on a chill roll at 65° C. The resultingfilm was 43 um thick.

EXAMPLE 2

This example was made identically to example 1 except the chill rollspeed was slowed down to thicken the film to a thickness of 76 μm.

Optical Testing

The two samples from above were tested optically to determine if theyperformed as reflective polarizers. The test equipment used was aintegrating sphere attached to a spectrophotometer (Perkin Elmer Lambda650S). A reflectance is determined at a wavelength of 55 0nm. A lighttransmission value is also obtained at a wavelength of 550 nm. In orderto determine the performance of the films as a reflective polarizer,polarized light was first directed onto the films with the polarizationbeing perpendicular to the machine direction of the film. The percentageof light transmitted is measured as Tmax. The percentage of lightreflected is measured as Rmin. Next, polarized light was directed ontothe films with the polarization being parallel to the machine directionof the film. The percentage of light reflected is measured as Rmax. Thepercentage of light transmitted is measured as Tmin.

Table 1 shows the results of the above described optical testing on theexample films. In order for the films to demonstrate performance as areflective polarizer Tmax should be greater than Tmin and Rmax should begreater than Rmin. The larger these differences are the higher theperformance of the film as a reflective polarizer.

TABLE 1 Example Tmax Tmin Rmax Rmin 1 86.1 72.9 25.1 13.5 2 80.1 66.529.9 18

It can be seen in Table 1 that indeed both samples show a degree ofreflective polarization. It is expected that upon further optimizationthe performance could be significantly improved to even higherperformance.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention. The entire contents of the patents and otherpublications referred to in this specification are incorporated hereinby reference.

PARTS LIST

-   1—composite sheet-   10—circular fiber-   11—elliptical fiber-   12—rectilinear fiber-   20—continuous phase-   30—fibrils/discontinuous phase-   31—elliptical fibril-   32—rectilinear fibril-   33—triangular fibril-   40–ordinary refractive indices in X and Y directions-   41—extraordinary refractive index in Z direction-   50—extruder barrel-   60—extruder exit

1. A fiber comprising a birefringent fibril discontinuous polymericphase dispersed in a continuous polymeric phase wherein the refractiveindices of the discontinuous and continuous phases in the X and Ydirections are substantially matched to each other, the refractiveindices of the discontinuous and continuous phases in the X and Ydirections are substantially mismatched from the refractive index of thefibril discontinuous polymeric phase in the Z direction, and wherein theextrusion melting temperature of the continuous phase is less than theonset melting range of the discontinuous phase.
 2. The fiber of claim 1wherein the cross-sectional shape of the fiber is circular, rectilinear,elliptical, triangular, tri-lobal, or trapezoidal.
 3. The fiber of claim1 wherein the cross-sectional shape of the fiber is circular orelliptical.
 4. The fiber of claim 1 wherein the birefringent fibrildiscontinuous polymeric phase has a cross-sectional shape that iscircular, rectilinear, elliptical, triangular, tri-lobal, ortrapezoidal.
 5. The fiber of claim 1 wherein the birefringent fibrildiscontinuous polymeric phase has a cross-sectional shape that iscircular or elliptical.
 6. The fiber of claim 1 wherein the birefringentfibril discontinuous polymeric phase comprises a polyester.
 7. The fiberof claim 6 wherein the polyester comprises polyethylene(terephthalate) ,polyethylene(naphthalate) , or any copolymers of either.
 8. The fiber ofclaim 6 wherein the polyester comprises polyethylene(naphthalate). 9.The fiber of claim 1 wherein the continuous polymeric phase comprises apolyester, an acrylic, or an olefin.
 10. The fiber of claim 1 whereinthe continuous phase comprises polyethylene(terephthalate),poly(methyl-methacrylate), poly(cyclo-olefin), or any copolymers ofeither.
 11. The fiber of claim 1 wherein the number of fibrils in thefiber is greater than
 50. 12. The fiber of claim 1 wherein fibrils eachhave a cross sectional area of less than 3 square microns.
 13. The fiberof claim 1 wherein the ratio of discontinuous phase to continuous phaseon a weight basis is less than 2 to 1
 14. The fiber of claim 1 whereinthe fiber has been cold drawn at a temperature just above the Tg of thefibril polymer to achieve a high level of birefringence of thediscontinuous phase.
 15. The fiber of claim 14 wherein the fibertemperature is between 2 and 20° C.
 16. The fiber of claim 14 whereinthe fiber has been cold drawn at least 2 to
 1. 17. A process for makinga diffusely reflective polarizer film comprising the steps of: a)providing fibers that comprise two components distributed as (1) acontinuous phase and (2) a discontinuous phase in the form of fibrils,respectively, wherein at least the phase 2 is birefringent; b) cuttingthe fibers to a desired length; c) extrusion melting the cut fibers at atemperature sufficient to melt phase 1 but not melt phase 2; d)extrusion mixing the melted phase 1 with the unmelted phase 2 at thesame time as step c) or after step c) to uniformly disperse the phase 2fibrils as a mixture within phase 1; e) subjecting the dispersed mixtureto shear forces whereby the fibrils of phase 2 become oriented withinthe mixture; and f) forming a composite film from the mixture comprisingthe oriented phase 2 fibrils.
 18. The process of claim 17 wherein thediscontinuous phase in the form of fibrils has a cross-sectional shapethat is circular, rectilinear, elliptical, triangular, tri-lobal, ortrapezoidal.
 19. The process of claim 17 wherein the discontinuous phasein the form of fibrils has a cross-sectional shape that is circular orelliptical.
 20. The process of claim 17 wherein the discontinuous phasecomprises a polyester.
 21. The process of claim 20 wherein the polyestercomprises polyethylene(naphthalate).
 22. The process of claim 17 whereinthe continuous phase 1 comprises a polyester, an acrylic, or an olefin.23. The process of claim 17 wherein the continuous phase 1 comprisespoly(1,4-cyclohexylene dimethylene terephthalate) orpoly(ethylene-terephthalate/isophthalate) copolymer.
 24. The process ofclaim 17 wherein the fibers that comprise two components have greaterthan 1000 phase 2 fibrils.
 25. The process of claim 17 wherein the phase2 fibrils each have a cross sectional area of less than 0.2 squaremicrons.
 26. The process of claim 17 wherein the refractive indices ofthe discontinuous and continuous phase in the X and Y directions of thefibers that comprise two components are substantially matched, therefractive indices of the discontinuous and continuous phase in the Xand Y directions of the fibers that comprise two components aresubstantially mismatched from the refractive index of the discontinuousphase of the fibers in the Z direction and wherein the extrusion meltingtemperature of the continuous phase is less than the onset melting rangeof the discontinuous phase.
 27. An optical element comprising a filmcontaining a layer including continuous phase and discontinuous phasematerials, wherein the discontinuous phase materials are cut fibrils andinclude a material having a different refractive index in the orthogonalX and Y directions in a plane perpendicular to the direction of lighttravel.
 28. The optical element of claim 27 wherein the cut fibrils arecut to a length of less than 5 mm.
 29. The optical element of claim 27wherein the cut fibrils are cut to a length of less than 1 mm.
 30. Theoptical element of claim 27 wherein the cut fibrils are cut to a lengthof less than 0.4 mm.
 31. A display comprising the optical element ofclaim
 27. 32. The display of claim 31 further comprises at least onefunction selected from the group consisting of image viewing screen,antireflection layer, ambient light suppression, color filter array,light valve, illumination enhancement, light collimation, lightdirecting, light diffusion, stiffening, resistance to thermal expansion,light spreading, a light source, image algorithm, image storage, imagebuffer, optical brightener, IR reflection and a power source.